This distinction between what counts as truth in philosophy (generally speaking) and in Christianity is significant. Plato, famously in the Theatetus, has his interlocutors worry the notion that knowledge is “justified true belief,” which of course they do so well that by the end of the dialogue they (and the reader) are left to start over, though now the path has been cleared a bit. For Christianity, however, knowledge is communion. Truth that is an object of intellection is not identical to the truth that is a Person.
If this is so, then it follows that the organs for knowing such respective truths are likely different. The intellect seems an appropriate enough organ for apprehending those truths that are objects of such intellective energies. But what of the apprehension of truth that is Personal? Here Christianity is clear: the organ for such knowing is the heart.
There is no need, I do not think, to belabor the point that the intellect is concerned about its own matters: the various intellective objects which may occupy its sphere of review and influence. For ancient philosophers, this was perhaps a bit more wide-ranging than modern philosophy takes to be the case. But, and this cannot be stressed enough, it is the case that even these diverse objects were still objects of the intellect.
Persons, however, are not subject to being objects of thought. Persons are not capable of being effectively conceptualized. Once that is done, one no longer contemplates a person but a concept. But that something may not be conceptualized does not lead to the conclusion that it may not be known. That is to say, we speak of knowing mathematical truths. We also speak of knowing persons. But the respective knowledge is different.
Leaving aside epistemological arguments regarding the possibility of genuine knowledge, if such is possible generally, that we do have true knowledge of persons is as true as we have knowledge of objects of thought. It is not here our purpose to compose an epistemology of love. Rather it is important to note not only the distinction between heart and intellect, and their respective venues for knowledge, but, to give some shape to the notion that knowledge of the heart is in fact knowledge.
We will give shape to this I think if we do so by some analogy to logic, however distant may be the relation. That is to say, in the Christian view, as persons are logoi of the Logos, it would not be improper to say that there is a sense of personal logic to persons. This “logic” however is not susceptible to intellective conformation, or, rather, it necessarily exceeds intellective conformation. As has been said, the heart has reasons which reason does not know. If there is an analogue within intellective energies to this personal knowing, it may be something like the Aristotelian nous, in which the intellect grasps principles in an intuitive look as wholes. Here, this knowing of persons grasps the knowledge of a person as a whole, whatever data there may be in terms of habits, preferences, historical background, etc; and grasps this whole as a mystery not subsumable to rational comprehension. Rather, in terms of knowing persons, we are on the surer ground of the organ of the heart.
This heart by which we know persons should not be schematized in a dialectic of opposition to the intellect. This is a particular modernist prejudice. It is knowledge in the same degree that intellective knowledge is knowledge, though it is qualitatively different, of course. Indeed, as I will remark in a moment, there is overlap between the two spheres of knowledge. It is not a syllogistic knowledge, of course, but it is not without its structure and form.
Indeed, when considered under something like an Aristotelian rubric, one can see that even with regard to intellective knowledge, what is generally considered knowledge under modernist paradigms is more closely aligned with what Aristotle calls episteme, an organized body of data, or scientific knowledge. What Aristotle calls techne also could be said to have its organization, but it is knowledge that is generally handed down via tradition from master to apprentice. Similarly, phronesis also could have its organization, but again, as it deals with the analyzing of choices in the flow of action, this, too, is communicated not so much by analytical texts (the Ethics notwithstanding), but in the way of life shared by friends within an ordered construct of a particular social context.
So, it is not unreasonable to consider that the knowledge of the heart is not without its own logic, organization, content, means of dissemination, and so forth. It is not, however, limited to or defined by analytical constraints.
Now, while philosophy itself quite intentionally limits truth to intellective objects, the Christian understanding of truth is much broader and more inclusive. In fact, if in Christian thought and experience truth is quintessentially a Person, then the intellect is much too narrow and much too limited a capacity for knowing such truth. Rather, for the intellect to experience its fulfillment, its greatest end, it must be moved within the heart that it, too, may experience this Person who is truth, and though such a Person will infinitely far exceed the capacity of the intellect’s knowledge, what the intellect is able to know will be strengthened and fulfilled by this greater and more extensive knowledge as the intellect finds its proper context for knowing.
The problem here, between philosophy and Christianity, which again tends to widen the two instead of bridging them, is that what is understood to catalyze knowledge in each sphere is presumably different. Philosophy understands itself as the accurate grasp of reality through the intellective energies of reason. Christianity understands itself as the accurate experience of reality through faith. But even the ancients understood that faith is not opposed to reason. In fact, reason itself understands the need for faith for it to work.